Added missing changelog.

Author: depryf

CHANGELOG.md

@@ -3,6 +3,7 @@
 **Changes in version 5.9 (not released yet)**
 
 - Changed format of version for NHIA and NAPIIA and set both values to "20240801" and "20250801" respectively.
+- Optimized multiple algorithms to use sets instead of lists when looking if a value is contained in a collection (no changes in behavior).
 - Added optional behavior and DX year input to the Cause of Death Classification utility methods. This won't affect the corresponding algorithm, which uses the 2008 SEER Site Recode logic.
 
 **Changes in version 5.8**